I have exatly the same problem well described by alejandro.
My system is a gentoo amd64, xorg 7.1.1, kde 3.5.4.
The problem appear even in non composited desktop but is linked to the rate of the movie: when the action slow down, non distorsion appear, but when screen flash or move fast the distorsion came back, alway at the same place.
I was thinking the problem was related to the diffrence between the movie display rate and my LCD display rate (I have 2 LCD both @60Hz), so that the problem came from the player.
Please tell me if the problem belong to the driver or not.